The present invention relates to a zoom lens for use in a still camera. More particularly, the invention relates to a very compact zoom lens that has an f number of approximately 3.5 to 4.5 and which attains a zoom ratio of about two ranging from a half viewing angle of about 32.degree. at the wide-angle and to about 17.degree. at the semi-telephoto end.
Two general types of zoom lens are known that attain a zoom ratio of about two ranging from a wide angle to an angle at the semi-telephoto end: one type is generally referred to as the two-component type consisting of a negative first lens component and a positive second lens component and is disclosed in Unexamined Published Japanese Patent Application Nos. 64811/1984 and 142515/1984; and the other type which is shown in Unexamined Published Japanese Patent Application Nos. 97016/1983 and 111013/1983 is composed of a negative first lens component, a positive second lens component, and a negative third lens component which is fixed behind the second lens component (this latter type of zoom lens is hereunder referred to as the third-component fixed type).
Numerous zoom lenses of the two-component type are known, and the lens system shown in Unexamned Published Japanese Patent Application No. 64811/1984 is characterized by the shorter overall physical lens length that is realized by providing an aspherical surface within the first lens component and by suitably composing said component of two lens elements. However, since the first lens component has a small power and must be moved by a large amount during zooming, the overall lens length at the wide-angle end is by no means small. In addition, because of the great lens extension that is needed for focusing purposes, the closest focusing distance cannot be reduced satisfactorily. The system shown in Unexamined Published Japanese Patent Application No. 142515/1984 does not employ an aspherical surface and has a greater overall physical length than the system shown in Unexamined Published Japanese Patent Application No. 64811/1984. A further problem with the two-component type is that, compared with the third-component fixed type having the same overall lens length, the required movement of the second lens component having a stop diaphragm cannot be sufficiently reduced (as will be described hereinafter).
The zoom lens of the third-component fixed type is a rather recent concept but is essentially the same as the combination of the zoom lens of the two-component type with a rear converter. The system disclosed in Unexamined Published Japanese Patent Application No. 97016/1983 has the advantage of small distortion but, probably because of the fact that it is intended for use in a 6.times.4.5 cm or 6.times.7 cm camera, this lens system has a relatively large overall length and has to employ many lens elements, particularly in the first and third lens components. The system shown in Unexamined Published Japanese Patent Application No. 111013/1983 has the advantage of compactness but, because of the excessively great negative power of the first and third lens components, the system experiences great variations in spherical aberration and astigmatism during zooming. Additionally, the excessive power of the first lens component results in a need to move the second lens component to a comparatively large extent in spite of the presence of the third lens component.
Instead of being fixed, the negative third lens component may be designed to be movable toward the object side together with the second lens component as shown in many parents such as Unexamined Published Japanese Patent Application Nos. 26754/1979 and 59157/1979, but compared with the third-lens component fixed type, this movable type has a large overall length and requires a large movement of the second lens component having a stop diaphragm.